The present invention relates to electrical connectors for use with flat multi-conductor ribbon cables, and particularly to connectors compatible with high-speed data transmission.
In modern electronic systems, such as computer systems, flat multi-wire cables (ribbon cables) are commonly used to carry signals to and from printed circuit boards, disk drives, and the like. As the speed of data transmission in electronic systems increases, the problem of crosstalk between adjacent signal wires of the ribbon cable increases dramatically. Crosstalk results from electromagnetic interference and can cause reduced signal clarity, including damage to the integrity of the data being transmitted.
To combat the problem of crosstalk, the preferred connector and ribbon cable assembly for high-speed signal transmission is a configuration that provides a ground-signal-ground (G-S-G) wire configuration. In this manner, signal wires are separated by at least one ground wire, which thereby reduces or eliminates crosstalk and enables higher speed signal transmission without significant signal degradation.
In a conventional connector, each wire of the ribbon cable is connected to a signal contact in the connector, and desired signal contacts (and associated wires) are grounded. Thus, in a conventional connector each ground wire requires a corresponding grounded signal contact within the connector body. This arrangement reduces the number of contacts in the connector available for signal transmission. In other words, to provide the preferred ground-signal-ground wire configuration, about one-half of the contacts in the connector are dedicated for grounding, thus leaving only the remaining one-half of the contacts in the connector for signal transmission. This means that for a required number of signal lines, the number of contacts in a connector (and thus the size of the connector) must be increased to accommodate the additional ground lines, unless a novel connector design is developed.
The art is replete with connectors for terminating ribbon cables. As is common in the electronics industry, standardized connector specifications (often referred to as form factors) have evolved for specific applications to ensure the compatibility of components which originate from different manufacturers. A well known standardized connector used with ribbon cables is the AT attachment (ATA) interface. The ATA interface, as originally designed, utilizes a 40 pin connector and is used to terminate a 40 conductor ribbon cable. The wire configuration of the original ATA interface is not ground-signal-ground, meaning that the signal wires are not separated by ground wires. Thus, the original ATA design is susceptible to crosstalk as data transmission speeds increase.
As the electronics industry has continued to develop systems that operate at higher and higher speeds, the ATA interface has been pushing the limits of reliable performance at transmission speeds of 33 Mbytes/s. Because the introduction of devices with transmission speeds of 66 Mbytes/s and higher is imminent, changes to the ATA interface standard have been made by the Small Form Factor (SFF) Committee. The new SFF standard, SFF-8049, retains the existing 40 pin connector interface, but replaces the original 40 conductor ribbon cable with an 80 conductor ribbon cable. The new SFF standard requires that the ground connections all occur within the same 40 pin form factor of existing ATA connectors. To retain the existing size and grounding configuration of the 40 pin connector, the 80 conductor ribbon cable has a pitch of 0.025 inch, which is one half of the 0.050 inch pitch of the earlier 40 conductor cable. This means that the distance between the wires of the 80 conductor cable is only one half of the distance between the wires of the 40 conductor cable. Alternating wires of the 80 conductor ribbon cable are connected to the grounded signal contacts of the ATA interface such that ground wires are interposed between each signal wire to minimize crosstalk and the associated signal degradation. Obviously, because only 40 signal contacts are present in the connector, each grounded conductor cannot have its own grounded signal contact. Therefor, new and unique connector designs are required to ground the additional wires of the 80 conductor ribbon cable within the 40 pin connector form factor.
Others have developed connectors for this purpose. For example, Circuit Assembly Corporation of Irvine, Calif., USA, manufactures a connector (available under the part number CA-40ATAS-C-X01) which utilizes a ground bus located inside the connector between the two rows of traditional insulation displacement contacts (IDC). A schematic representation of a portion of the Circuit Assembly connector is shown in FIG. 1. The ground bus utilizes insulation displacement contacts 10 placed on a 0.050 inch pitch to mass-terminates every other conductor of the 80 conductor ribbon cable. The grounded signal contacts 12 of the 40 pin ATA interface are electrically connected to the ground bus by conductive wipers 14 which extend between the ground bus and the grounded signal contacts 12. While the Circuit Assembly connector performs adequately, it has several disadvantages. The primary disadvantage is that the wiper contact between the ground bus and the selected grounded signal contacts is susceptible to damage during the assembly process and performance degradation over time due to surface corrosion which develops at the wiper-contact junction. Further, because the ground bus is inserted into connector housing between the signal contacts, molding the housing and assembling the components is difficult, due to the close proximity of the housing cavities which receive the signal contacts and the ground bus. The Circuit Assembly connector also has the disadvantage of being dedicated to a specific wire configuration. That is, if it is necessary to change the signal/ground wire configuration (for example, due to a form factor alteration or for a different connector application), an entirely new housing must be molded and a new ground bus designed with relocated wipers for making electrical connection to the connector contacts.
AMP Incorporated of Harrisburg, Pa., USA, manufactures a connector similar to the above described Circuit Assembly connector. Like the Circuit Assembly connector, the AMP connector (available under the part number 120605-X) utilizes a ground bus located inside the connector between the two rows of traditional insulation displacement contacts (IDC). A schematic representation of a portion of the AMP connector is shown in FIG. 2. The ground bus uses insulation displacement contacts 20 to mass-terminates every other conductor of the 80 conductor ribbon cable. In addition, the ground bus also terminates the wires associated with the grounded signal contacts 22. This means that at certain locations the AMP ground bus must terminate adjacent wires of the 80 conductor cable, which are spaced only 0.025 inch apart. This presents a disadvantage for the AMP ground bus, because it is very difficult to manufacture insulation displacement contacts at such a small pitch. As a result, certain design compromises must be made which decrease the reliability of the connections made by the ground bus. For example, as the pitch of the IDC features decreases, the amount of material available to form the IDC decreases, which in turn can affect the compliance of the IDC. If the compliance of the IDC is inadequate, the connection may fail over time as thermal cycling of the connector occurs. An illustration of an insulation displacement contact 30 for use with conductors 32 having a 0.050 inch pitch is shown in FIG. 3A, while an illustration of an insulation displacement connector 34 for use with conductors 36 having a 0.025 inch pitch is shown in FIG. 3B. The IDC 34 of FIG. 3B is less compliant than the IDC 30 of FIG. 3A, due to the presence of conductor 36', which reduces the ability of IDC arms 37 to flex. The IDC 34 of FIG. 3B is thus not as reliable and more susceptible to damage than the IDC 30 of FIG. 3A. It is therefor preferred to avoid the use of insulation displacement connectors like that shown in FIG. 3B.
Adaptability of the connector design is especially important in light of the new ATA interface standard. The ATA interface and cable assembly connects the motherboard to two devices, which could, for example be a disk drive or a CD-ROM drive. The original standard allowed any connector to be connected to either device or to the motherboard. This is not true with the revised standard, which requires that the grounding scheme of each connector be individualized to a specific location, such that the connectors for the motherboard, the first device, and the second device cannot be substituted for each other.
Therefore, it would be very beneficial to provide a connector which reliably grounds alternate wires of a fine pitch ribbon cable, which also allows selected wires associated with the connector signal contacts to be grounded, which is easily manufactured and assembled, and which is readily adaptable to differing wire configurations at minimal cost.